Method for producing alkali metal alcoholates

ABSTRACT

The invention relates to a method for producing alkali metal alcoholates by reacting alcohol with alkali metal in an aprotic, organic solvent in the presence of an H acceptor such as e.g. isoprene, butadiene, styrene or methyl styrene.

This is the National Phase Application of PCT application Ser. No.EP99/03188, filed May 10, 1999.

The invention relates to a method for the preparation of alkali metalalcoholates, in which method alcohol is reacted with alkali metal in anaprotic organic solvent using a hydrogen acceptor.

Alkali metal alcoholates R-OM (R=alkyl, M=Li, Na, K, Rb, Cs) arecompounds that are susceptible to hydrolysis and are often used inorganic synthesis on account of their basic properties.

It is known that alkali metal alcoholates can be prepared by reactingthe corresponding alcohols with an alkali metal in accordance with thereaction equation:

2R—OH+2M→2R—OM+H₂

R=alkyl

M=Li, Na, K, Rb, Cs

The speed of this reaction diminishes as the length of the alkyl chainincreases and also as branching increases. Whilst the reaction can besuccessfully accelerated to a considerable extent on a laboratory scaleas a result of the use of extremely finely divided alkali metal, whichis produced with a particle size below 50 μm by means of high-speedstirrers, the reaction takes many hours on an industrial scale ofproduction. Long reaction times, however, impair the economic efficiencyof this alcoholate synthesis.

A method for preparing alkali metal alcoholates is known from FR-PS 1070 601, in which alkali metal is finely distributed, in a boiling inerthydrocarbon by being stirred, and after cooling the calculated quantityof alcohol is added drop by drop to the dispersion. During thepreparation of the sodium suspension, any agglomeration of the finelydistributed sodium is prevented by adding a dispersive additive, such asfatty acid, surfactants or active carbon. The alkali metal alcoholate inxylene that results can be separated.

With the method known from DE-OS 34 37 152 for the catalyzed preparationof alkali metal alcoholates from alkali amalgams and alcohols, lumps ofanthracite are used as the catalyst, the surface of which is preferablycoated with a mixture of nickel and molybdenum oxide. Aliphatic alcoholshaving 1 to 4 carbon atoms are preferably used.

With the method known from DE-PS 08 45 341 for the preparation of alkalimetal alcoholates that are lean in caustic alkali, amalgamated alkalimetal is brought into contact with alcohol several times in the presenceof catalysts, such as graphite. In this connection, it is further knownfrom DE-PS 09 28 467 that finely distributed sodium amalgam and alcoholcan be directed in counterflow with respect to a lumpy catalyst,consisting of a mixture of graphite or active carbon and metal filings.

The methods set out above have the following disadvantages:

The reaction times cannot be successfully reduced in an economicalmanner on an industrial scale of production by means of the use of lumpyor filing-like catalyst substances.

The necessary separation of the catalysts from the reaction product isproblematic in many cases.

The use of a toxic amalgam compound as the alkali metal component isproblematic on account of the impact on the workplace and environment.

Since the speed of reaction is often simply insufficient unsatisfactorywhen elemental alkali metal is used, in particular in the case ofsterically hindered tertiary alcohols, despite the measures described,very basic organometal compounds have also been used as the alkali-metal source. This holds good in particular for the preparation oflithium alcoholates:

R—OH+R′Li→R—OLi+R′H↑

The disadvantage of this smoothly running reaction is the comparativelyhigh price of organo-lithium compounds.

Further syntheses are based on alkali metal hydrides and amides. Whilstthese reagents often react somewhat faster or clearly faster than thealkali metal in elemental form, the compounds, calculated on a molarbasis, are clearly more expensive than the alkali metal. In the case ofthe amides, moreover, ammonia develops that has to be removed from thewaste-gas stream at a cost. When hydrides are used—compared with the useof the elemental metals—twice the quantity of hydrogen develops. Whilsthydrogen is not an ecologically hazardous product, the resultant gasstream is loaded with organic compounds (solvent, alcohol) which, forecological reasons, as far as possible should not reach the environment.

R—OH+MH→R—OM+H₂↑

 R—OH+MNH₂→R—OM+NH₃↑

R=alkyl residue; M=alkali metal

The object of the present invention is to avoid the disadvantages inaccordance with the prior art, that is, in particular to set forth amethod for the preparation of alkali metal alcoholates that starts withinexpensive raw materials that are available commercially and which in avery rapid reaction supplies water-free alkali metal alcoholates whilstforming as few gaseous by-products as possible and without using solidcatalysts that are difficult to separate.

The object is achieved in that the alcohol is reacted with the alkalimetal (Li, Na, K, Rb, Cs) in an aprotic organic solvent, and anH-acceptor in the form of a conjugated diene or a 1-arylolefine ismoreover added thereto. The reaction preferably proceeds in accordancewith the following reaction scheme:

The presence of an H-acceptor brings about an advantageous reduction inthe quantity of waste gas, from the point of view of reaction-controland environmental-protection, in comparison with the conventionalreaction of alkali metal alcoholate formation.

Open-chain or cyclic, unsubstituted or substituted 1,3-dienes orunsubstituted or substituted 1-arylolefines can be used as theH-acceptors (in the case of the substituted reagents, both in the cisand in the trans form). Preferred H-acceptors for this reaction areisoprene, butadiene, cyclohexadiene-(1,3), styrene or methyl styrene.

The quantity of H-acceptor added amounts to 0.2 to 4 times, preferably0.4 to 1.5 times, the stoichiometric quantity, that is, 0.2 to 4 mol,preferably 0.4 to 1.5 mol, relative to, in each case, 2 mol alcohol. Themethod can consequently even be carried out with the quantity ofH-acceptor that is added lying below the stoichiometrical relationship,this increasing the economic efficiency.

In particular one of the metals Li, Na or K or mixtures of these metalscan be used as the alkali metal.

It is advantageous that the alkali metal can be present in pulverulentform, granular form or lumps. In the case of Na, K, Rb or Cs in additionpreferably a finely divided molten mass can be chosen. On account of itshigh melting point, lithium is preferably used in a solid form.

In particular in the case of the reaction of secondary or tertiaryalcohols with an alkali metal, the presence of an H-acceptor results inclearly higher speeds of reaction in comparison with methods knownhitherto. The reactions of i-propanol, t-butanol or t-pentanol are ofparticular commercial interest.

An aliphatic or aromatic hydrocarbon with 4 to 20 C-atoms or an ether ora mixture of these substances can be used as the aprotic organicsolvent. The reaction can be carried out particularly well in hexane,heptane, octane, toluene, ethyl benzene, methyl-tert. butyl ether(MTBE), tetrahydrofuran (THF) or 2-methyl-THF. Commercially availablehydrocarbon mixtures, such as, for example, petroleum ether, paraffinoil, high-boiling Shellsol D 70, can also be used in a particularlyadvantageous manner as the solvent.

The mixture of alcohol and H-acceptor is preferably added to thedispersion of alkali metal in the aprotic organic solvent. It is alsopossible to produce a mixture of solvent, H-acceptor and metal, to whichthe alcohol is added in doses. In some cases, it is also possible to addthe alkali metal in a solid or liquid form to the mixture of solvent,alcohol and H-acceptor.

A solution of lithium tert-butylate in THF can be prepared in this way,for example.

The temperature of reaction is maintained at −20 to 200° C., preferablyat 20 to 140° C.

The subject-matter of the invention is explained in greater detail inthe following with reference to exemplifying embodiments.

EXAMPLE 1 Synthesis of Sodium Tert-butylate (STB) in Toluene in thePresence of a Stoichiometric Quantity of Styrene

4.78 g (208 mmol) Na-lumps were placed in 69.8 toluene in a 250-mlfour-necked flask with a heating mantle, precision glass stirrer andreflux condenser, and heated to approximately 100° C. At the start ofthe addition of a mixture of 15.0 g (202 mmol) tert-butanol and 10.5 g(101 mmol) styrene, the temperature of the reaction mixture wasimmediately increased to 109° C. During the addition which took place in25 minutes, no significant development of gas could be observed. Towardsthe end of the addition, small quantities of a white deposit could beobserved on the flask wall, although this disappeared 5 minutes afterthe end of dosing (clear, light yellow solution). The temperature wasmaintained at approximately 100° C. for a further 25 minutes.

The product solution was filtered in the hot state and dried in arotation evaporator until the weight was constant. 18.2 g (94%) STB inthe form of a colourless powder was obtained.

Comparative Example A Synthesis of Sodium Tert-butylate in TolueneWithout an H-acceptor

Using the same apparatus as in Example 1, tert-butanol was dosed into aboiling Na-dispersion (containing 61.5 g sodium =2.7 mol) in toluene.After the addition of 70 g tert-butanol (35 mol %, relative to theNa-quantity that is used), 17% of the quantity of hydrogen to beexpected in theory had been formed after 1.4 hours only, suggesting thatthe speed of reaction was substantially slower than that of Example 1 inaccordance with the invention.

EXAMPLE 2 Synthesis of Lithium Tert-butylate in Tetrahydrofuran in thePresence of a Stoichiometric Quantity of Isoprene

9.62 g (1386 mmol) lithium granules (Na-content 0.43%) were placed in400 g THF in a 1 l double-jacket reactor with a reflux condenser, dripfunnel and precision glass stirrer, and a mixture of 106 g (1430 mmol)tert-butanol and 47.2 g (693 mmol) isoprene was added thereto at 15 to35° C. The reaction started immediately at the beginning of the 45minute dosing time; intensive cooling was necessary. After a 30 minutesecondary reaction time, only a very small quantity of Li-metal wasstill present in an otherwise clear, slightly grey-coloured productsolution. 2.35 mmol/g total base concentration (corresponding to a 95%reaction) were detected in a sample of the solution. After one furtherhour at 30° C., the base concentration rose to 2.43 mmol/g(corresponding to 98%). Throughout the reaction time, no significantdevelopment of hydrogen was observed.

It was possible to filter the product solution in a problem-free manner(glass frit, filtration time approximately 1 minute).

Comparative Example B Synthesis of Lithium Tert-butylate inTetrahydrofuran Without an H-acceptor

3.5 g (0.5 mol) Li-granules (0.77% Na-content) were suspended in 100 gTHF in a 500-ml flask with a reflux condenser, drip funnel and precisionglass stirrer, and a solution of 37.1 g (0.5 mol) tert-butanol in 60 mlTHF was added thereto under reflux within 145 minutes. During this time,28% of the H₂-quantity that was to be expected in theory was formed.After a further 3 hours boiling under reflux, a filtered sample wastaken and tested for the total base (2.04 mmol/g corresponding to an 82%reaction).

After cooling, filtering was carried out by way of a glass frit. Thefiltration process took 100 minutes and yielded a cloudy, yellowsolution. The reaction time is also longer in this comparative examplethan in Example 2.

EXAMPLE 3 Synthesis of Potassium Tert-amylate (PTA) in Hexane at 60° C.in the Presence of a Stoichiometric Quantity of Isoprene

25.3 g (648 mmol) of purified potassium crusts (Merck) were melted in305 g hexane in a 0.5-1 twice clad reactor with a precision glassstirrer, reflux condenser and drip funnel, and then a mixture of 57.1 g(648 mmol) tert-amyl alcohol and 22.1 g (324 mmol) isoprene was addedthereto at approximately 60° C. within 100 minutes. During the additionprocess, the reaction mixture became slightly cloudy, although was easyto stir. At the end of the addition, refluxing was effected for 10minutes, with a clear, slightly yellowish solution being formed.

The filtered solution was concentrated by evaporation in a rotationevaporator under vacuum. 74.8 g (92%) of a colourless powder wasobtained that had the expected composition for PTA.

EXAMPLE 4 Synthesis of Lithium Tert-butylate in THF in the Presence of aSub-stoichiometric Quantity of Isoprene

5.2 g (0.75 mol) lithium metal granules (0.4% Na-content) were suspendedin 300 ml THF in an apparatus like that in Example 3, and a mixture of16.2 g (238 mmol) isoprene and 97.3 g (772 mmol) tert-butanol was addedthereto within 3 hours. The reaction temperature was maintained atapproximately 40° C. At the end of dosing, the mixture was stirred againfor approximately 1 hour at 40° C.; afterwards no metallic lithium couldbe detected any more. The slightly cloudy solution was filtered withoutany difficulties by way of a glass frit (filtration time approximately30 sec). A base concentration of 2.18 mmol/g was analyzed in theslightly grey-coloured, yet clear filtrate, this corresponding to ayield of 98%.

During the reaction phase, approximately 170 mmol H₂-gas had beenformed, that is, approximately 45% of the metal reacted directly withthe alcohol. In this Example, the quantity of isoprene used correspondedto 0.62 times the stoichiometric quantity.

What is claimed is:
 1. Method for the preparation of alkali metalalcoholates by reacting alcohol with alkali metal in an aprotic organicsolvent, characterised in that the synthesis is carried out in thepresence of an H-acceptor, with the H-acceptor being an open-chain,unsubstituted or substituted 1,3-diene

with R₁, R₂=H, alkyl, vinyl (R₁, R₂ in cis or trans position) or acyclic 1,3-diene

with n=1 to 5 or an unsubstituted or substituted arylolefine

with R₃, R₄=H, alkyl (R₃, R₄ in cis or trans position).
 2. Methodaccording to claim 1, characterised in that the H-acceptor is isoprene,butadiene, cyclohexadiene-1,3; styrene or methyl styrene.
 3. Methodaccording to claim 1, characterised in that the quantity of H-acceptorused is 0.2 to 4 mol per every 2 mol alcohol.
 4. Method according toclaim 3, characterised in that the quantity of H-acceptor used is 0.4 to1.5 mol per every 2 mol alcohol.
 5. Method according to claim 1,characterised in that one or more of the metals Li, Na or K is or areused as the alkali metal.
 6. Method according to claim 1, characterisedin that the alkali metal is present in solid form (pulverulent form,granular form or in lumps) or, in the case of the Na, K, Rb or Cs, evenin a finely divided, liquid form.
 7. Method according to claim 1,characterised in that a secondary or tertiary alcohol is used as thealcohol.
 8. Method according to claim 7, characterised in thati-propanol or t-butanol or t-pentanol is used as the alcohol.
 9. Methodaccording to claim 1, characterised in that an aliphatic or aromatic C₄-to C₂₀-hydrocarbon or an ether or a mixture of the substances mentionedis used as the aprotic solvent.
 10. Method according to claim 9,characterised in that hexane or heptane or octane or toluene or ethylbenzene or methyl tert-butyl ether or diethyl ether or tetrahydrofuranor 2-methyl tetrahydrofuran is used as the aprotic solvent.
 11. Methodaccording to claim 9, characterised in that a hydrocarbon mixture, suchas petroleum ether or high-boiling hydrocarbon mixtures (paraffin oil),is used as the aprotic solvent.
 12. Method according to claim 1,characterised in that a mixture of alcohol and H-acceptor is added tothe alkali-metal dispersion in a solvent.
 13. Method according to claim1, characterised in that the temperature of reaction is maintained at−20 to 200° C.
 14. Method according to claim 13, characterised in thatthe reaction temperature is maintained at 20 to 140° C.
 15. Methodaccording to claim 1, characterised in that possible excess alkali metalis filtered off, and an alkali metal alcoholate solution is obtained asan end product.
 16. Method according to claim 1, characterised in thatat the end of the reaction possible excess alkali metal is filtered off,the reaction solution is concentrated by evaporation, and solid alkalimetal alcoholate is obtained as an end product.